Disposable automatic lancet is the main trend of today's lancet device, which has been improved and developed over years. The Chinese patent office authorized an invention patent on Apr. 7, 2010 with the title of “SAFE AND CONVENIENT DISPOSABLE AUTOMATIC LANCET”, the patent number of 200810136675.3, and the authorized publication number of CN101444428B. This patent utilizes the fit of a protective rod and the internal space of an ejection chamber to allow a needle core to be set in a locked state of to-be-launched, and before use, due to the blocking effect of an end of the protective rod, the button cannot be pressed down and then the needle core is set in a safety state. When in use, the needle core can immediately enter a launching state once the protective rod is removed by screwing, and after launching, it automatically enters the locked state by locking the button through the fit of the button and housing, and cannot be used again. Compared with other disposable automatic lancets, this patent has advantages of advanced technology, easy to operate and safe to use. However, in the specific solution given in Example 2 of this patent (see its FIGS. 12 to 15), after the protective rod is removed, 11.5 mm length of the needle body of the needle core is exposed, which has a problem that the exposed length of needle is too long. Too long exposed length of needle not only increases the force of screwing the protective rod, but also results in a biggest defect that the needle is easily to bending deflected during puncture. Particularly, with the development of medical device technology, the required blood volume of the existing blood testing equipments becomes smaller and smaller, and in order to reduce algesia, the diameter of the lancet is made finer and finer, i.e. changing from the former 23 G (Φ 0.6 mm) and 26 G (Φ 0.45 mm) to the present 28 G (Φ 0.36 mm) and 30 G (Φ 0.30 mm). Such a fine lancet, if it has too long exposed length of needle, is prone to bending and deflecting during puncture, which greatly reduces the puncture performance of the needle tip. Accordingly, the object of the present invention is how to solve the problem of too-long-exposed-length of needle, and on the other hand, retain the existing safe and convenient design.